Heat-resisting austenitic steel alloys



F. EISERMANN EI'AL 2,772,155

HEAT-RESISTING AUSTENITIC STEEL ALLOYS Filed Oct. 14. 1953 Per Cent h i 21 1 7 1 3 9 :5 J

P c M4 21 c c b'ned fl-ifi 'rio er en D an 0 0m a INVENTORS Fk/ 50m cw ESERMANM WA LTEE S/EcFR/ED.

ATTORNEY United States Patent M HEAT-RESISTING AUSTENITIC STEEL ALLOYS Application October- 14 1953, Serial No. 386,006

Claims priority, application Switzerland October 18, 1952 Y 6 Claims. (Cl. 75128) The present invention relates to heat-resisting austenitic steel allows which, although particularly suitable for castings, in comparison with other steel alloys of about equally high temperature stability are considerably lower in secondary components that are expensive and often diflicult to obtain in the desired quantities.

The graph shown in the accompanying drawing illustrates one particular feature of the present invention.

Heat-resisting steel structures can be produced from rolled and forged material or by means of steel castings. Due to the fact that many heat-resisting materials, especially those designed for stresses at high temperature, are difficult to work, the use of steel castings has lately increased.

It has been demonstrated, however, that most of the steel alloys which hitherto have been used in the production of rolled or forged material, are not suitable for the production of castings. More specifically, it has been found that the existing alloys, when cast, lack the favorable mechanical properties they possess when rolled or forged and that castings made therefrom do not satisfy the necessary requirements.

Steel intended for rolling or forging is usually deposited as ingot steel in iron molds, whereas steel for steel castings is poured into refractory molds. At the transition from the liquid into the solid state, ingot steel as well as cast steel passes through a stage of primary crystallization, i. e. a formation of crystals at a well-defined arrangement within the melt, which follows definite rules. In the case of stable austenitic steel these primary crystals remain in existence during the cooling, down to room temperature. In addition, secondary phases occur in the mass at room temperature, which can be distinguished as either phases that'separate out above the solidus line; that is to say that they cannot be eliminated by means'of a solution treatment, or phases that come in existence below the solidus line and which, depending upon the temperature, can be completely re-incorporated in the ferrous mass by means of a solution treatment. V

Themethods employed in further processing ingot steel and cast steel difier insofar, as ingot steel is subjected to a hot or cold'plastic deformation step, whereby a crushing and scattering of the secondary phases takes place as Well as a unilateral stretching of the primary crystals which produces the well-known lineal structure.

Cast. steel, on the other hand, retains the secondaryphases and the primary crystals if not subjected to a special heat treatment (solution treatment). The result is a dissimilarity in the mechanical properties of rolled and cast materials because of their different structure, whereby in steel intended for heat-resisting castings a suitable formation of the secondary phases and the size of the austenitic grains is of importance.

percent of nickel, cobalt and manganese.

2,772,155 Patented Nov. 27, 1956 In contrast to cast steel, because of the subsequent working, even an unfavorable 'austenitic grain size or secondary phase formation in forged or rolled steel can still result in satisfactory properties of the work piece.

As far as steel castings are concerned, a solution treatment, which leads to the diminution or even the complete disappearance of the secondary phase, cannot be applied because of the danger of deformation of the work pieces while they are cooled down from the 'necessarily high solution temperatures. On the other hand, rolled and forged material, as a rule, can be subjected, prior to further work, to a solution treatment suflicient to cause disappearance of all phases below the solidus line:

The requirements which must be observed in connection with castings from plain steel apply to a still greater extent in the case of steel alloys cast in hot molds, such as, for example, in precision casting. In this instance, particular forms of secondary phases may develop because of differences in the cooling conditions and these secondary phases may be of considerable influence upon the heat resistance of the cast material.

The heat-resisting austenitic steel alloys according to the invention, which are particularly suitable for eastings, have the following generic composition:

Carbon 0.05 to 0.45 percent.

Chromium 16 to 27- percent.

Nickel 10 to 30 percent.

Cobalt, manganese 6 to 12 percent at approx. equal parts.

Molybdenum, 'tung- 6 to 8 percent, the proportional sten, columbium/ amounts being 3+ 0.8 percent tantalum. molybdenum, 2+ 0.8 percent tungsten: 1+ 0.8 percent columbium/ tantalum.

Steel alloys of the herein disclosed type may comprise the usual proportions of silicon, phosphorus and sulfur. In order to increase the corrosion-resistance, in certain cases a silicon content of up to 2 percent is preferred. These steel alloys may additionally contain 0.1 to 0.2 percent nitrogen. In order to. improve their resistance at high heat, boron may be added ,in an amount of 0.4

percent and up to 1 percent at the most. In the presence of nitrogen, a heat resisting steel according to the invention preferably comprises a total amount of 25- Moreover, such steel alloys are preferably subjected to an aging process after casting; for example by delaying the cooling of the casting. Furthermore, it is advisable to limit the carbon content of alloys according to the invention to such a proportion that'no continuous lattice structure of eutectic phases occurs.

- Example 1 A steel of the following specific compositionthas been found to be particularly suitable:

ing the proper precipitation effect;

Castings, produced from steel alloys of the foregoing composition prove to have an endurance of 324" hours 7 until rupture, at a test load of 23 kg./sq. mm. and a a.

test temperature of 700C. 7

7 Notwithstanding that columbium and tantalum, as type related elements, always occur together, in a steel according to the presen'tinvention, they may be present singly or jointly. 1

Itfis known that heat of the re-c'rystallizationtemperature of the ferrous mass,

upon the amount and degree of precipitation of the inter- "calated'secondary phases'and upon the micro-dispersion ,7 phases present in the mass. Taking the foregoing facts. 7 1 inconsideration; various steel alloys may be' developed according to this invention fromfthe following basic composition: i a r g I Percent Carbon; 15 Chromiumj Nickel V 25; Nitrogen Since any increase in the recrystallization temperature of the mass andthe attainment of the desired precipitation V v eifect' are primarily the resultof the presence of the elements molybdenum, tungsten and colum bium/tantalum, i. e., the type' of elements which narrow the dissolving range, the nickel content is" adjusted 'to a proportion of 25- percent in order toobtain astable austenitic composition The chromium content depends upon the required creased by an addition of silicon up to 2 percent. The

addition of nitrogen to the basic composition is for the purpose of keepingthe nickel ratio 'down'and for-attain The preferred way" of proportioning the quantitative 4 a r 7 values which are many times those of the steel samples e, fand'g, although the last three samples deviate only to a numerically small degree in the relative amounts of these elements.

7 After the above-described findings, attempts were-made;

= to produce a further increase in the endurance value,

resistance depends upon th'e' level 10 .Thus, in a'first series of tests, part of thernickelcontent, P

was replaced by cobalt which was added in increasing amounts, up to a maximum of '10 percent. In anotherte st series, partof the nickel of the'basic masswas replaced by manganese, likewise in increasing amount s'up to 10 percent.;/These attempts did not produce an'a'ppre ciable imprdveme'ntvwith respect to high temperatureresistance. Q

, However, a surprising improvement in heat endurance; p is obtained it-partofthe nickel is substituted by-about 'equfal parts ofcobaltand-manganese, at a, proportion of corrosionresistance which, if'desired, can be further inamounts 'of the supplementary elements .mdlybdenum,

' tungsteni andQcolumbium/tantalum is to always 'keep the" suniot' these elements 'con'stanuatnot less than 4 percent, but pre ferably'at dpercent. Moreover, it is .to be borne in mind-that the 'carbon content of the; alloy must be 7 also balanced with j respect to 'these supplementary -ele merits.v 'Asthe results or tests conducted with steel alloys compounded according to .theselimitations, it'lwas found that even1srn all changesrin the proportion of thesaid supplementary elements produce exceedingly great changes in thehe'at resistance of; the alloys.

The following tabulation illustrates theendurance qualiqti'es of castQsteelsIcompi-ising'the aforernentioned basic ferrous mass anda'total content of 6 percent of the s'up- V plementary elements molybdenum, tungsten and colum-.

bium/tantalum., The figures given indicate the endurance 7 time at ait'est temperature of 700 Ciand a test loadof 23 kg.'/sq."mm.: a I Y "Boron Percent ratio of:' s Time until 7 Steel 7 I rupture T (inhours) -Mo:W:Cb/'Ia.

. -l.5 'l 72 2 1 i V :84- 2 1.5 s2 1.2 2 re 6' 1 2. 10, 2 2.5 1' 2. 5 1O The'ehtffurahce values in the 'rar'e ejingtabmati qath w i the decisive i nfluence of the relative amounts of "these supplenie'ntary elefiinir'upen the heat resistance; of": 7 various alloys comprising the same basic mass: Thus, the.

steelsamples bib, 2:, containing pfoportional amounts of I molybdenum-tungsten an iumbiiifii'iiant'atiim' in ac! cor'danee t6 the hereinclaimed'invenuion, have endurance 1 An important property of -suchalloys rs,

' flow of 'liquidsteel increases fwith-increasingcar on';co'n'; 7

7 On thepthenhand, higher amountsflof carbon. 1

,may produce ametwork ofjeutecticphases with increasing: V numbers of interconnectin'g links which; may cause 'a- 'de crease in the high-temperature resis'tance tent.

6' to 'l2 percent of the combined'elements. 1 Thus, the steeldescribed in Example l'ghas an' endurance-value of 324 hours until rupture, at a test'temperature of 700" V C.and atestload of 23 kgl/sq rnmr The accompanying graph-ilIustratesItl ie results'of'replacing various jamou'nts of the 25 percent of nickel by equal amountsof' cobalt' and'rnang'anese 'combinedand' testing the steel under the saidconditions (700? CI, 23 k'g./sq: mm). Theendurance times (inhour's) are plotted 'on the logarithmic scale of the ordinate while the abscissa indicates the Co/Mn content percent. The sum of the amounts of cobalt and; manganese 'isjequal to the amount of replaced nickel and the poi'nt ofrir itersection of the two coordinates indicates a nickel content of- 25 percent. The plotted curve shows a 7 clearly pronounced maximum in the'endu'rance until rup-. ture if in an, alloy ofthe herein claimed type 8'to 12 percent of the nickel are replaced by 4 too percent of each, 7 cobalt and manganese.

EJtampIeZ 7 'A ifurther unexpected improvement in the high-temperatureresistance,ofthese alloys results from the. addition of-boron, in an amount front 0'.'4 to 1.0, percent;

.Thus, a steel comprisingaccording'to this inventio'n:

p 'Pe'rcent ani'endurance:time of l lthours untilrupture. 31 8 g v ltest.temperaturesof 700 v -C. and atest load of 23; kg.-/sq. rnm.-

. f It is; expedient 1 in Imany'; instances to; subiect steel alloysfofihis invention'to an'ragingltreatmentf This can;

be accomplished: by eitherjheat treating; the ,castjin a furnacelorjas, a; result of exposure to a normal operationtemperature zo'f the device'gor ma chine whichcomprises;

thec stw; Anoth rwar i ac n st sin xt a;

the cooling period afterthe pastingr f f the degree offluidity -in feasting. It is known that the que sy-.1

In a steel of this invention good flow properties can be obtained simultaneously with the desired resistance at high temperatures by means of restricting the carbon content to such an extent that no continuous lattice structure of eutectic phases occurs.

The use of a steel according to this invention is not restricted to the production of cast articles. Steel of the herein disclosed composition can be rolled and forged for the subsequent production of heat resisting construction elements.

Having described the invention, what is claimed for the purpose of obtaining Letters Patent is:

1. A heat-resisting austenitic steel alloy particularly suitable for casting articles capable of withstanding severe mechanical stress at high temperatures, said steel alloy containing, a ferrous base, essentially 0.05 to 0.45 percent carbon, 16 to 27 percent chromium, to 30 percent nickel, a total of 6 to 8 percent molybdenum, tungsten and columbium/tantalum, the three last named constituents being. present in the following proportions: 2.2 to 3.8 percent molybdenum: 1.2 to 2.8 percent tungsten: 0.2 to 1.8 percent columbium/tantalum, and a total of 6 to 12 percent cobalt and manganese in substantially equal amounts, and incidental impurities.

2. A heat-resisting austenitic steel alloy as defined in claim 1, containing silicon, phosphorus, sulfur, nitrogen, and boron in the conventional proportions.

3. A heat-resisting austenitic steel alloy according to claim 1, containing 0.1 to 0.2 percent nitrogen and the combined amount of nickel, cobalt and manganese being substantially 25 percent.

4. A heat-resisting austenitic steel alloy according to claim 1, containing boron in an amount of 0.4 to 1.0 percent.

5. A heat-resisting austenitic steel alloy, containing in a ferrous base, essentially Percent Carbon 0.15

Silicon 0.8 Manganese 4 Chromium 18 Nickel 17 Cobalt 4 Molybdenum 3 Tungsten 1.5 Columbium/ tantalum 1.5 Nitrogen 0.15

6. A heat-resisting austenitic steel alloy, containing in a ferrous base, essentially Percent Carbon 0.15 Silicon 0.8

Manganese 4 Chromium 18 Nickel 17 Cobalt 4 Molybdenum 3 Tungsten 2 Columbium/ tantalum 1 Nitrogen 0.15 Boron 0.6

References Cited in the file of this patent UNITED STATES PATENTS 2,504,453 Rotherham Apr. 18, 1950 2,516,125 Kramer et al July 25, 1950 OTHER REFERENCES Wartime Report, National Advisory Committee for Aeronautics, Advance Report 40 22, pages and 71. Edited by Freeman et al. Received at the Patent Office Library Nov. 26, 1948. 

1. A HEAT-RESISTING AUSTENITIC STEEL ALLOY PARTICULARLY SUITABLE FOR CASTING ARTICLES CAPABLE OF WITHSTANDING SEVERE MECHANICAL STRESS AT HIGH TEMPERATURES, SAID STEEL ALLOY CONTAINING, A FERROUS BASE, ESSENTIALLY 0.05 TO 0.45 PERCENT CARBON, 16 TO 27 PERCENT CHROMIUM, 10 TO 30 PERCENT NICKEL, A TOTAL OF 6 TO 8 PERCENT MOLYBDENUM, TUNGSTEN AND COLUMBIUM/TANTALUM, THE THREE LAST NAMED CONSTITUENT BEING PRESENT IN THE FOLLOWING PROPORTIONS: 2.2 TO 3.8 PERCENT MOLYBDENUM: 1.2 TO 2.8 PERCENT TUNGSTEN: 0.2 TO 1.8 PERCENT COLUMBIUM/TANTALUM, AND A TOTAL OF 6 TO 12 PERCENT COBALT AND MANGANESE IN SUBSTANTIALLY EQUAL AMOUNTS, AND INCIDENTAL IMPURITIES. 